This invention relates to data display terminals.
A data display terminal is a device used to encode, display, and send data to a remote data processing system, and to receive and display data from the data processing system. Such display terminals have many applications. For example, the display terminal can be located in a stock broker's office and can provide for the display of stock market data supplied over teletype lines from a remote computer on inquiry from the display terminal.
The television monitor has for a variety of reasons become the preferred device for displaying the data. In operation, the monitor displays a page of characters arranged in rows and columns with each character being made up of selected bright dots within a dot matrix. Conventionally, the dot matrix is 7 .times. 5 (i.e., 7 raster lines by 5 dot positions). Control of the video is achieved by converting in a predetermined sequence a plurality of character-representing data words into a pattern of pulses that control the intensity of the television beam during the course of its raster scan. Thus, the video control signal must be an appropriately synchronized serial-by-bit format.
Whereas only one bit of information at a time is provided to the monitor, the display terminal must provide storage for many thousands of other bits of information making up the data words for the full page. Moreover, the page storage means must be accessible not only for reading in connection with video control but also for writing and reading in connection with entry and removal of data.
The page storage means has data entered into and removed from it so that it maintains an up-to-date record of data received from the remote computer and from a keyboard included in the data display terminal. Various arrangements for the page storage means have been proposed, and various approaches have been directed to the control task of addressing and timing accesses to it so that data can be properly located in it and accessed on a non-interfering basis.
In one conventional arrangement, the page storage means is constructed as a group of recirculating shift registers, each such shift register providing storage of data words corresponding to a row of characters. Although this arrangement to some extent facilitates the task of providing a serial-by-bit video signal format, the digital circuitry necessary to implement the addressing function is quite complex. This is particularly so with respect to editing. For example, in the operation of a data display terminal, it is sometimes necessary to rearrange the display of characters by moving some characters from one row to another. This of course means that corresponding data words in some part of one shift register must be transferred to some part of another shift register. The timing and control circuitry required to achieve this is generally quite complicated and accordingly increases the overall cost of the data display terminal.
The page storage means has also been constructed in the form of a random access, location addressable memory. According to this approach, each character-representing data word is stored in a different addressable location of the random access memory. Several problems have arisen in connection with this type of memory. First of all, the problem of conflicting demands for access to the memory is significant. That is, at a time when the random access memory is tied up in connection with a read access for purposes of forming the video control signal it is not available for access in connection with entry of new data words. One approach to this problem is to disable the read accesses related to the video control signal whenever new data words are to be entered. However, when this is done, the display screen will go blank for an interval of time and this can be disturbing to the operator.
A separate problem involved with the use of such random access memories relates to the addressing system for them. Inasmuch as the page of characters is arranged as a row/column matrix, it is preferable that the random access memory storing the data words for the characters to be addressable in terms of a row/column address. So long as the number of characters per row is a power of two (e.g., 16, 32, 64, etc.) this poses no problem. Accordingly, the known data display terminals that have employed a random access page memory (rather than the shift-memory-type page memory) are organized so that the number of characters per row is such a power of two. In many cases, however, so as to present a better composition of text on the display screen, it is preferable to have a different number of characters per row.